Telephone system



Aug. 25, 1936. c. E. LOMAX TELEPHONE SYSTEM Filed NOV. 9, 1935 mmmbhsas mmxu kmkz 4INVENTOR. CLARENCE E. LOMAX Patented Aug. 25, 1936 UNITED STATES TELEPHONE SYSTEM Clarence E. Lom'ax, Oak Park, Ill., assignor to Associated Electric Laboratories, Inc.', Chicago, 111., a corporation of Delaware Application November'i), 1935, Serial No. 48,977

3 Claims.

The present invention relates to telephone systems in general, but is concerned more particularly with automatic telephone systems. The object of the invention is the production of a novel arrangement for improving the reliability of the testing operation of an automatic switch when connection is being extended to a desired called line.

The invention is concerned particularly with the provision of a satisfactory solution of a problem which involves automatic connector switches The nature of the problem to be solved will be apparent from the following brief discussion:

In an automatic telephone system, it is customary .to so arrange the circuit apparatus that a negative potential is placed on the third or control conductor of a subscriber line by way of the individual cut-off relay thereof when the line is idle and to place a grcundpotential on this conductor to operate the cut-off relay when the line is in use, which ground potential is employed in a testing operation to indicate the fact that the line is in use.

For-several years, a standard testing arrangement has been used in connection with twomotion Strowger connector switches. This testing arrangement includes a double test, first for the presence or absence of the guarding ground potential on the third or control conductor of the called line, and then for an idle-indicating battery potential in the event that the ground potential is not encountered. The sequence of these two tests is controlled by a slow-acting changeover relay which is energized throughout the rotary movement of the selector. The test for the ground potential is effected by' a' busy relay which operates and locks itself in the event that ground potential is encountered by the test wiper when the switch has come, to rest, while the final test is accomplished by the usual wiperswitching'relay, to the test winding of which the test wiper is transferred upon the restoration of the above-mentioned changeover relay. In the event that the called line is busy, the busy relay locks itself, as above mentioned, upon the restoration of the slow-acting relay and the rotary stepping circuit. is opened by the cooperation of the busy relay and the slow-acting changeover relay so as to prevent further unnecessary advance of the wipers in the event that the calling subscriber should dial an additional digit, as well 5 as when the calling subscriber replaces his receiver.

When the called line is idle, the above-mentioned wiper switching relay is permitted to operate following the changeover voperation. As is necessary because of the relatively small amount of power available for the operation of the wiperswitching relay in its test circuit (over the test wiper in series with the cut-off relay of the called line), the first operation of the switching relay is to close a pair of lightly adjusted locking contests in order to produce a local energization of the relay sufficiently strong to bring about a complete operation. A short time elapses, therefore, following the closure of the locallocking circuit before the disconnection of the rotary magnet occurs upon the full energization of the relay. Therefore, if the calling line is momentarily interrupted an additional time and this interruption is so timed that the line relay of the connector transmits an additional impulse after the locking circuit of the switching relay has been closed and before the rotary magnet has been disconnected, the connector is caused to take an addi- .tional step and the wipers of the connector are advanced to the next succeeding line; Connection is made with the next succeeding line whether it is busy or idle, as the local locking citcult of the switching relay remains established. Under this condition, therefore, the connector makes connection with a busy line; this is highly undesirable, for it enables the privacy of a telephone conversation to be invaded. v

In addition to permitting the privacy of a conversation to be invaded, the foregoing occurrence often leads to the called busy line being tied up and withheld from further service because of a locked condition which results when a connector cuts in on a busy line when the connection is reversed between the connector banks and the llneswitch, as will be brought out in the detailed description which is to follow.

The additional impulse which causes the undesired operation above referred to is believed ,to be usually transmitted by a, momentary depresslon of the hook switch at the calling substation almost immediately following the delivery of the last impulse of the final digit by the calling device. This momentary depression of the hookswitch may be inadvertent and without any v malicious intent on the part of the calling subscriber, or (as. has been found to be the case) intentional and malicious on the part of informed groups of subscribers. One of these informed subscribers, on calling a line and finding it busy, often replaces the receiver and then removes it and again dials a number the second time setting the connector on the preceding contact group,

in the bank. If the line associated with the preceding contact group is idle, the switching relay starts to operate in the usual way and closes the locking circuit previously mentioned, but the additional impulseis then received and causes the wipers to be stepped to the busy line before the switching relay has operated far enough to disconnect the rotary magnet, whereby connection with the busy line is obtained, because the switching relay (having once closed its local locking circuit) operates fully regardless of any subsequent test conditions which may be encountered.

Once the problem previously, set forth has been discovered and analyzed, several simple remedies may suggest themselves, such as arranging that the reoperation of the slow-acting control relay which occurs when the additional impulse is delivered shall break the locking circuit of the switching relay unless the relay has operated sufiiciently to open the rotary magnet circuit. An arrangement such. as this, however, involves a major wiring disturbance of the switch, as additional spring contacts must be added to the existing relay apparatus, which usually proves to be quite expensive. The invention is concerned with the production of an arrangement curing the trouble by the addition of new equipment to the connector and without making an extensive change in the existing equipment.

In the preferred embodiment of the invention,

rotary ofi-normal contacts are provided which by a very simple wiring change cooperate with the slow-acting control relay above mentioned to open the rotary stepping circuit upon the restoration of the slow-acting control relay, and

therefore before the switching relay has had time the usual circuit diagrams a sumcient amount.

of the equipment in a telephone system embodying the invention to enable the invention to be understood. Fig. 1 shows the apparatus involved in a call between two lines in a typical telephone system employing a two-motion Strowger reiease=by-last-party connector C to which the invention has been applied, while Fig. 2, shows the invention applied in its modified form.

In Fig. 1 the substation A is a well known form of automatic substation provided with serially related transmitter and receiver, and the usual form of calling device; a lineswitch of the usual rotary type is indicated at LSl; a selector of the usual Strowger type is indicated at S; at C the circuits of a connector are shown complete with the preferred to of the invention applied aosaosi thereto; at B is shown an automatic'substation similar to the substation A, and at LS2 the circuits of the lineswitch associated with substation B are shown for illustrative purposes.

In Fig. 2 the self-locking relay 5! is provided in lieu of the rotary-ofl-normal contacts ll of Fig. 1, as will be pointed out hereinafter.

Detailed description The disclosure havingbeen described in general, a detailed description of the operation of the apparatus shown will now be given. For this purpose, the regular operation of the system will be first described as it takes place without the invention applied to the connector C. Previous to the application of my novel invention thereto, the connector C is as shown, with the wire indicated by the dotted line is in. place and with resistor l8 and rotary-ofi-normal contacts ll not included. In the detailed operation which is to follow, it will he parts ii and it are not applied to the connector C and that the wire leis in'its normal position as indicated in the drawing.

A call from substation A to substation B Assuming now that the subscriber at substation Adesires to converse with the subscriber at substation B the subscriber at substation A removes his receiver and then dials the digits of the desired number. With the system as herein indicated (there being one selector and a connector in the switchtrain) a three-digit number is used.

When the receiver is removed at substation A, the lineswitch LS5 operates in the usual maer to select an idle trunk extending to a first selector, the one extending to the selector 8, for example. f

When the first digit is dialled, the selector S raises its wipers opposite the desired level and then rotates them over the contacts in this level in search of an idle connector. it may be es= sumed that the connector first one found to be idle, in which'case this connector is seized by the selector 5.

When the connector 5' is seized, line relay 9 is energized over the calling line icy way of the her mally closed contacts controlled by ermatures of the hack-bridge relay 5. Upon energizing, line relay E3 operates release relay '5 to prepare the selector for operation, which includes the placing of ground potential on the local locking conductor .2" that the 32 and the application of ground potential to release trunk conductor 8 in order to provide a holding circuit for the selector 8 and the lineswitch LS8.

When the calling subscriher dials the second digit in the desired number, each resulting deenergization of line relay 8 results in the momentary opening of the circuit i slow-acting release relay 7 and in the closure of a circuit for vertical magnet E3 by way of vertical series relay 3 through the inner lower store of the operated relay l. Vertical series relay 8 operates and makes a circuit transfer in order to maintain its own circuit and ti; of vertical magnet is intact after the ofimormal springs ll At the end or the vertical movement, vertical series relay 8 restores and transfers the operating c rcuit to rotary magnet l4 through contacts of busy relay l and the middle lower armature and resting contact of switching relay i2.

When the final digit is dialled, each resulting impulse from the contacts of line relay 6 is transmitted to rotary magnet l4. By the operation of magnet H the wipers 20-22 of the connector are rotated step-by-step until they come into engagement with the terminals assigned to the line of the desired substation, B. The slow-acting rotary-series relay I l is energized in multiple with rotary magnet i4 and remains energized throughout the series of impulses delivered to the rotary magnet. At its upper armature, rotary-series relay disconnects test wiper 2| from the test winding of switching relay l2 and transfers it through contacts of relay |2 to the busy-test relay III, while at its lower armature it places a shunt around the lower armature'and resting contact of relay so as to maintain the circuit of the rotary magnet and its own circuit intact against an opening thereof 'by busy relay |0 occasioned by energization of the busy relay responsive to test wiper 2| encountering grounded test contacts during the rotary movement.

The operations taking place at the end of the rotary movement depend upon whether the called line is busy or idle. If the called line is busy, busy relay I0 is in an energized condition when the wipers come to rest on the terminals of the called line, as test wiper 2| then encounters a grounded test contact. In this event, busy relay i0 has prepared a self-locking circuit, which is completed when rotary-series relay restores upon the cessation of impulses. With relay restored and relay ill in operated condition the test circuit through the upper winding of relay I2 is opened at the upper contacts of relay III; the rotary magnet circuit is opened at the inner lower armature of busy relay l0 and the lower contacts of the now-restored rotary-series relay H; and a busy-tone signal from busy-tone lead 45 is transmitted back over the lower talking conductor to the calling subscriber to inform him that the called line is busy. The calling subscriber is expected to replace his receiver and permit relays 6 and I to restore pursuant to the Y restoration of the connector C and the clearing out of the partly established connection.

Assuming now that the line of substation B is idle when called, busy relay i0 is not in an energized condition when the wipers come to rest on the terminals of the called line. In this event, the rotary-magnet circuit is still unopened when rotary series relay restores upon the cessation of impulses, and a test circuit is completed from the grounded incoming release-trunk conductor 2 to test wiper 2| by way of the upper winding of switching relay 2. which test circuit includes the upper contacts of' relays ill and ii. The upper winding of switching relay I2 is now energized in a circuit which includes wiper 2|, the switching relay 46 of the lineswitch LS2, and the winding of the operating magnet 48. Relays l2 and 48 are energized in this circuit, but, as is well known,

magnet 48 is not operatively energized. Relay 44 operates to disconnect the windings of line relay 46 and ground from the talking conductors of the called line, but the relay is not completely operated to connect up the wipers because of the usual interlocking arrangement (not shown) between the armature of relay 46 and the armature of relay 41.

In the connector 6, when relay I 2 is partially energized by means of its upper winding, it first nects up the wipers 2|! and 22 to prepare the talking circuit and close the ringing circuit; and it applies ground potential by way of contacts of closes a local locking circuit for itself at its ini ring-cut-ofi relay 23 to the motor-start conductor 43 so as to start the ringing apparatus (not shown) into operation. H

Ringing current from the interrupted-generator lead 44, superimposed upon ground potential, is now applied through the lower contacts of I relays 9 and i2 and one conductor of the called line. As is well known, relay 9 is so retarded in action that it does not respond to the flow of ringing current in the event that bridged ringing is employed, and direct current is prevented from flowing by the usual condenser in series with the ringer 'at the called substation.

When the called subscriber removes his receiver' to answer the call, relay 8 operates by means 01' its upper winding and closes a locking circuit for itself at its inner lower armature to conductor 22, grounded at the lower armature of release relay I. Relay 9 also opens the ringing circuit and completes the talking circuit.

At this point, back-bridge relay 5 energizes over the called line, a ground connection having been applied to its lower winding by the closure of the locking circuitv of the lower winding of relay 8 from grounded conductor 32. Upon energizing, back-bridge relay 5 reverses the connections between the incoming talking conductors 2 and 4 and the windings of line relay 8 so as to provide a reversal of current flow in the calling line for whatever purpose desired. Back-bridge relay 5 also disconnects ground potential from supervisory-one conductor 42 at its lower armature, which ground potential is applied upon the operation of ring-cut-oir relay 8. At the same lower armature of back-bridge relay 5, a new source of ground potential is applied to conductor '32 so as to maintain relays 9 and I2 locked independent of the continued operation of release relay 1, this feature having to do with the last-party-release operation of the connector C. to be subsequently explained. Backbridge relay 5 also opens an additional point in the circuit of release magnet l5, also having to do with the last-party-release operation, and prepares to apply ground potential to supervisory-two ccnconductor 4| upon the restoration of relays 4 and I.

Conversation may now proceed as desired.

In the event that the called subscriber is the first to replace his receiver, the restoration of the back-bridge relay 5 results in the extension of ground potential at the lower armature to supervisory-one lead 42, which may be utilized by a slow-acting signal device common to a group of connectors to indicate that a connector is being held from the calling line after the called subscriber has replaced his receiver. The connector is not released, because release relay 1 remains operated.

On the other hand, in the event that the calling subscriber is the first to replace his receiver, the deenergization of line relay 8 is followed by the restoration of release relay 1. Rotary magnet it is not operated at this time during the interval required for release relay l to restore after its circuit is opened by relay 6, as the rotary magnet has been disconnected at the contacts of switching relay l2. Rotary series relay Ii, however, has not been disconnected as it is now restored to perform an additional operation. When relay ll operates, it again makes a transfer of test wiper 2! from the winding of relay ii to the contacts associated with the windings of busy relay it, but busy relay it is not operated at this time as it has been disconnected at the inner ,upper contacts of switching relay IE to prevent its operation under the condition now obtaining. The operation of relay ii at this point serves merely to disconnect the incoming release trunk conductor 3 from test wiper 2i now grounded at the middle upper armature of relay i2, whereby the incoming release trunk conductor 3 becomes completely ungrounded upon the restoration of release relay 7.

At this point, the selector S is permitted to release and the lineswitch L8! is permitted to clear out.

The restoration of release relay I does not result in the ungrounding of conductor 32, and the consequent restoration of relays 9 to l2 because conductor 32 is now maintained grounded through operated contacts of relays l2, 9, and 5.

Whenrelay 1 restores, it opens the operating circuit extending to rotary series relay ll, permitting this relay to restore after a slight interval, whereupon ground potential is reapplied to the incoming release-trunk conductor 3 from the grounded test wiper 2i (grounded at the middle upper armature of relay l2). This ground connection includes the pper contacts of relays ii and Ill, and the upper winding of switching relay l2. The resistance of the upper winding of relay I2 is sufflciently low to cause test conductor 3 to test busy to any selector (such as S) which subsequently rotates over the contacts of the connector C. The connector C is thus maintained guarded against further seizure as long as it is held operated from the called line.

Upon the restoration of relay I, back-bridge relay being still in an operated condition, ground potential is applied to supervisory-two conductor 4|, by means of which a signal, common to a group of connectors may indicate that a connector is being held from the called line after the calling subscriber has replaced his receiver.

It may be explained that the holding of the connector C from the called line at this time prevents a needless operationof the lineswitch LS2 which would otherwise take place in the event that the calling subscriber is the first to replace his receiver, and a first selector would be needlessly taken into use and again released when the subscriber at substation B replaces his receiver. The arrangement disclosed is one that has been in use for many years and its use is based on the theory that it is better to retain in use the switch which has already been occupied than it is to permit the lineswltch on the called line to search for and take another switch into use.

When the called subscriber now replaces his receiver, the restoration of back-bridge'relay 5 results in the removal of ground potential from conductor 32, thereby opening the locking circuit of relays l0 and I2 to permit these relays to restore. Ground potential is also removed from supervisory-two conductor 4|, and the ciracsacsi cuit of release gnet 953 is closed by way of contacts in the vertical-cfi-normal spring-combination it. By the operation of release magnet E5, the connector 0 is restored to position, whereupon the release-magnet circuit is.

Unstandcrdopemtica of the connector 0 The regular intended operation of the connector C having been described, it will be pointed out how the unstandard and undesirable operation of the connector C may take place in the event that the calling lineis momentarily opened first as the switching relay i2 is in the process of operating responsive to a successful test of an idle called line.

It will be recalled that when the called line. is idle, the busy test, which is performed with rotary series relay it still energized, is terminated upon the restoration .of relay ii and that the idle test is applied by the upper winding of relay i2 in the event that relay III has not energized to open the circuit of the upper winding of relay l2. 'It will be recalled also that the first operation performed'by switching relay i2 is, of necesslty, the closing of the local locking circuit at the inner lower contacts of the relay in order to supply sufiicient power for the complete operation of the relay. Now, as is well known, a measurable length of ti e is consumed in the operation of a relay follo ing the closure of a local locking circuit therefor. Therefore, in the event that the calling line is momentarily interrupted at about the time the locking circuit of relay I2 is closed, an additional impulse is delivered before rotary magnet II is disconnected at the contacts-of relay I2, and the wipers 20-22 are advanced an additional step. Relay ll is operated by this additional impulse to open the circuit of the test winding of relay l2, and to again close a circuit for busy relay Ill. The local locking circuit for switching relay l2, however, is not affected and the relay completes its operation. Busy relay III is disconnected ,at the contacts of relay l2,- as it must be in order to prevent its undesired operation in the event that the called subscriber is-the first to replace his receiver following conversation. Relay I l is shortly restored after the termination of the impulse. With relay l2 completely operated, the ringing circuit is completed to the newly contacted line.

If this line is idle, the subscriber thereon is signalled in the usual manner. On the other hand, if the newly contacted line is busy the talking bridge across the conductors serves as an operating path for ring-cut-oif relay 9, and this relay operates immediately to complete the talking circuit to the busy line.

In many installations of connectors such as is indicated in Fig. 1, the conductors leading from the banks of a part of the connectors to the called linesv are reversed, having to do with the giving of two-party tenninal-per-station,

party-line service. The conductors referred to are commonly called normal conductors. The

normal conductors by which the line of substation B is reached from the connector C are shown reversed in order to illustrate an especially undesirable feature that results from the described unstandard operation of the connector-C. When the line of substation B is in use as a result ofa call having been originated from this line, the lineswitch LS2 is then operated to set up a connection by way of a first selector and through another connector such as C. The reversed condition of the normal conductors associated with the line of substation B results in a locking circuit'being closed through the back-bridge relay 5 in series with the windings of the connector in use in the call which was previously originated from the line of substation B. In this case, the established connection is not cleared out when the parties thereto replace their receivers, and the connector C is tied up and withheld from service, as is substation B and the entire switch train used to establish the connection from the line of substation B to the line which was called therefrom. This condition is, of course, eventually discovered by means of the signals controlled over the supervisory leads 4| and 42, but it is very often not discovered until the subscribers involved have been annoyed considerably, both by. the intrusion upon the privacy of their conversation and the resulting tying up of their lines which follow.

The improved connector The remedial measures which have been taken to remove the described defect from the connector C will now be explained. The remedying of the defect consists in cutting the wire I9, indicated by a dotted line; in connecting the resistor I8 as shown; and in applying rotary-oif-normal contacts I! to the mechanical structure of the switch and connecting them in circuit as shown. The rotary-off-normal contacts I1 may be of any wellknown or desired mechanical structure such as is often used in connection with Strowger automatic switches. The only requirement is that they openwhen the wipers of the switch are given their initial rotary step and remain open until the wipers of, the switch have been released.

They are customarily operated by a longitudinal cam device gripped around the shaft of the switch and with which the insulated part of the leading spring of the combination I! is normally in contact.

With the connector C modified as shown in the drawing, as soon as the rotary movement has started and rotary-oif-normal contacts I'I-have opened, the continued closure of the circuit for rotary magnet I4 depends solely upon the continued energization of rotary series relay II. Therefore, whether the called line is busy or idle, as soon as relay II falls back, the rotary magnet circuit is opened (conductor I9 having been removed) for the rotary-oif-normal contacts I! are in an open condition. The current which can flow through the resistor I8 is insufficient to operate rotary magnet I4, as this resistor is applied merely for the purpose of pemiitting rotary series relay II to be reoperated when the calling subscriber is the first to'replace his receiver, as previously pointed out. At such time, rotary magnet I4 has been disconnected from in circuit with rotary series relay II-at contacts of switch ing relay I2, and the current which flows through resistor I8 is sufficient to operate rotary series relay II when it is not shunted by the relatively low resistance rotary magnet I.

When the line called by the modified connector C is busy,'busy relay I is in operated condition and the resistor I8 is disconnected completely as there is no occasion under this condition for providing what may be termed a leakage path for the .ip -ra -ionof relay 1 after magnet It has been disconnected from in circuit therewith. It will be understood, of course, that advantage is taken of the inner lower contacts of busy relay Ill because they happen to be already provided on the busy relay and that the circuit operation would be substantial y the same if resistor I8 and rotary off-normal contact I! were bridged directly around the inner lower contacts of rotary series relay II instead of by way of the inner lower armature and resting contact of busy relay III.

The modification of Fig. 2

The modified arrangement shown in Fig. 2 will now be explained briefly. The relay SI and the contacts shown to left thereof. are provided instead of the rotary off-normal contacts I! for '5I opens the initial circuit of the rotary magnet and rotary series relay I I, whereby the continued closure of the rotary magnet circuit depends upon the continued energization of rotary series relay II, as explained in connection with Fig. 1. The resistor I8 has the same function in Fig. 2 as it has when applied in connection with the rotary off-normal contacts I! in Fig. 1. Relay 5|, as shown has a copper collar on the armature end thereof to render it sufiiciently slow-operating to permit relay II to operate first..

What is claimed is:

1. In a two-motion connector switch, an operating conductor, a vertical magnet and a rotary magnet, means for delivering impulses over said operating conductor to the vertical magnet to impart the desired vertical movement to the connector, means effective for transferring the operating conductor from the vertical magnet to the rotary magnet at the end of the vertical movement, a pair of contacts included in circuit with the rotary magnet and means for opening them at the beginning of the rotary movement, a slow-acting relay and means for operating it for the duration of the series of impulses transmitted to the rotary magnet, and contacts on the slow-acting relay in multiple with the firstnamed contacts, whereby the rotary magnet circuit is maintained intact after the first-named contacts separate and is broken when the slowacting relay restoresupon the cessation of the rotary impulses.

2. In a vertical-and-rotary connector switch,

a rotary magnet for imparting rotary motion tothe connector switch after it has been operated in its vertical motion, an operating lead extending to the rotary magnet for enabling it to be operated by impulses, a self-locking relay associated with the rotary-magnet-operating lead and arranged to operate responsive to the first rotary impulse and remain locked thereafter, a slow-acting relay associated with the said lead and arranged to respond throughout a series of impulses and restore at the end thereof, and contacts on the two relays connected in parallel relationship, the contacts on the self-locking relay being normally closed and the contacts on the slow-acting relay being normally open, whereby the two relays cooperate to open the Q aesaom rotary magnet circuit upon the cessation of the series of impulses.

3. In an automatic connector switch having means to drive its wipers over the terminals of 5 lines responsive to directive impulses, means connected to the test wiper of the connector during the driving operation for testing encounteresi lines, means effective upon the termination oi the driving operation for disconnecting the said testing means, and means. simultaneously efiec tive for disabling the driving means. 

